In the pulverized coal boiler, it is requested to suppress the NOx concentration contained in combustion gas generated when the pulverized coal fuel is burned by the pulverized coal boiler and as a measure against it, a double combustion method is mainly used.
A pulverized coal boiler with the double combustion method applied to, for example, as disclosed in Japanese Patent Laid-open No. Hei 9 (1997)-310807, is structured so as to install a pulverized coal burner in the furnace of the pulverized coal boiler and an after-air nozzle on the downstream side of the burner, feed pulverized coal fuel and combustion air from the burner, feed only combustion air from the after-air nozzle, thereby burning the pulverized coal fuel.
And, firstly, in the combustion by the burner section of the pulverized coal boiler, air of a volume lower than the theoretical air ratio necessary for perfect combustion of the pulverized coal fuel is fed into the furnace from the burner to burn the pulverized coal in a state of insufficient air, and NOx generated by the combustion of pulverized coal by the burner in a reductive atmosphere is reduced to nitrogen, thus the generation of NOx in the combustion gas is suppressed.
However, in the reductive atmosphere, unburned components remain due to insufficient oxygen and CO (carbon monoxide) is generated. Therefore, next, to perfectly burn the unburned components and CO which are generated in the reductive atmosphere, from the after-air nozzle positioned on the downstream side of the burner, combustion air slightly more than the air volume which is a deficiency of the theoretical air ratio is fed to the furnace to burn the unburned components and CO, and combustion exhaust gas with the unburned components and CO reduced is discharged from the pulverized coal boiler.
In the double combustion method of the pulverized coal boiler disclosed in Japanese Patent Laid-open No. Hei 9 (1997)-310807, to greatly reduce the unburned components, it is required to promote the mixture of combustible gas of imperfect combustion rising from the burner and after-air fed from the after-air nozzle.
Therefore, in Japanese Patent Laid-open No. Hei 4 (1992)-52414, to promote mixture of combustible gas of imperfect combustion rising from the burner installed in the boiler and after-air fed from the after-air nozzle, an after-air nozzle having a structure that the flowing form of the injection flow fed from the after-air nozzle is adjusted so as to have both a straight flow and a swirl flow is disclosed.